Actor Shia LaBeouf has been threatened with legal action following allegations he plagiarized a graphic novel in a short film he directed.

The Transformers star was recently accused of basing his movie, Howard Cantour, on a book called Justin M. Damiano by Daniel Clowes. He subsequently issued a series of public apologies to the author, posting messages through Twitter.com and even hiring a skywriting plane to spell out his regret over the skies of Los Angeles.

However, the apologies have failed to appease Clowes' lawyers, who hit LaBeouf with a cease-and-desist letter on Tuesday, in which the skywriting stunt was branded "foolishness".

In the legal letter, which the actor posted on his Twitter.com page on Wednesday, a lawyer representing Clowes, Michael J. Kump, claims LaBeouf's legal team failed to respond to initial contact regarding the alleged plagiarism in Howard Cantour, adding, "We have been waiting since December 27 to hear how Mr. LaBeouf intends to make (it) right, but all that has happened is... foolishness such as Mr. LaBeouf's New Year's Day skywriting frolic that exposed Mr. Clowes to further ridicule."

Kump also alleges that LaBeouf has caused further troubles in a recent picture he posted on Twitter which showed a storyboard for a project called Daniel Boring, which the lawyer claims is a "rip off" of Clowes' comic strip David Boring.

The letter urges LaBeouf to "immediately cease and desist from any further acts of copyright infringement, trademark infringement, conversion, and other other intentional and malicious wrongs in connection with Mr. Clowes and his works".

It adds, "Your client is seriously out of control. He must stop his improper and outlandish conduct directed at Mr. Clowes and his works... Leave Mr. Clowes alone, and address and fix these problems immediately."

In a bizarre move Wednesday, LaBeouf posted the cease and desist order from Clowes and his lawyer to his Twitter account: